4. DENSITY PERTURBATIONS AND GRAVITATIONAL WAVES

By far the most important property of inflationary cosmology is that
it produces perturbations, in the form of both density perturbations
and gravitational waves. The density perturbations may be responsible
for the formation and clustering of galaxies, as well as creating
anisotropies in the microwave background radiation. The gravitational
waves do not affect the formation of galaxies, but may
contribute extra microwave anisotropies on the large angular scales
sampled by the COBE satellite
[14,
15].
An alternative
terminology for the density perturbations is scalar perturbations and
for the gravitational waves is tensor perturbations, the terminology
referring to their transformation properties.

In this article I will focus on the nature of the predictions from the
inflationary cosmology rather than detailed comparison with
observations, which was the focus for a series of lectures at this
Summer School by Pedro Viana
[16].